Compaq 219700-001 - ProLiant - 1500 Reference Manual page 141

D-8 Understanding Drive Arrays
Data Guarding (RAID 4)
Data guarding, also called RAID 4, assures data reliability while using only a
small percentage of the logical drive storage capacity. A designated, single
drive contains parity data. If a drive fails, the controller uses the data on the
parity drive and the data on the remaining drives to reconstruct data from the
failed drive. This allows the system to continue operating with slightly
reduced performance until you replace the failed drive.
Data guarding requires a minimum of three drives (two data drives and one
parity drive) in an array and allows a maximum of 14 drives (13 data drives
and one parity drive). For example, in an array containing three physical
drives, data guarding uses only 33 percent of the total logical drive storage
capacity for fault tolerance. A 14-drive configuration (13 data drives, one
parity drive) uses only 7 percent.
DATA
WAR2-052.AI, 9-2.EPS
Figure D-6. Data Guarding Stores redundant data [P] on a separate drive
The SMART-2DH Array Controller supports RAID 4 for backward
compatibility. Because RAID 5 provides the same benefits with higher
performance, Compaq recommends using RAID 5 instead of RAID 4.
Writer: Pamela King Project: SMART-2DH Array Controller Reference Guide Comments: 295469-002
DATA DATA
File Name: K-APPD.DOC Last Saved On: 2/27/98 12:06 PM
COMPAQ CONFIDENTIAL - NEED TO KNOW REQUIRED
PARITY
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P = Parity